Oct 2008

Anaerobic Digestion Biogas-potential Assay

Anaerobic digestion is a natural biological process that offers many environmental protection advantages. In the past, digesters were challenged by management difficulties and lack of profitability. Currently, there are six dairy farm digesters in Michigan, but the need for odor control, the income potential of the renewable energy digesters, and improvements in the technology and the understanding of operational principles are creating additional interest. This overview of anaerobic digestion on dairy farms includes a general cost analysis. Costs can often be offset by the recovery of byproducts when a digestion system is properly designed and managed.

Anaerobic digestion of animal manure is receiving much attention as a technology to reduce odors, manage nutrients, and produce renewable energy. This interest has been spurred by an influx of people into rural areas and the recent dramatic spikes in energy costs.

The use of on-farm digesters can be traced back to the 1950s. Prior to 2005, there were six farm anaerobic digesters in Michigan— five failed within 6 years and one remained operational for 21 years until it was shut down when the farm sold according to Michelle Crook, Michigan Department of Agriculture. Primary reasons digesters failed in the past were management difficulties and lack of profitability. Currently, there are six dairy farm digesters in Michigan with several additional projects under construction or planned. The need for odor control, the income potential that renewable energy digesters offer, and improvements in the technology and the understanding of operational principles are primary reasons for the renewed interest. Further information on anaerobic digestion projects in Michigan can be found at the State of Michigan website.

Anaerobic Digestion

The process of anaerobic digestion entails a community of microorganisms that first convert complex organic wastes to organic acids (such as acetic and propionic acids) and then the organic acids to biogas, containing primarily methane and carbon dioxide. Bio-produced methane is a renewable energy source that can be used in boilers, cleaned of impurities to enable insertion in natural gas lines, burned in a generator to produce electricity or simply flared if energy production is not economical. Using the methane as energy or flaring results in a net reduction of carbon dioxide equivalents and other greenhouse gasses enabling the sale of carbon credits. Agricultural methane offsets (carbon credits) are offered to farmers who collect and combust methane gas and thus prevent it from reaching the atmosphere. Further information on carbon credits and agricultural methane offsets can be found at the National Carbon Offset Coalition (www.ncoc.us).

The digester also produces digestate (material remaining after digestion is complete). Digestate is homogeneous, easily transferred through pipes, has few pathogens, and has an unassuming odor. This material can be managed in several ways. Separating the solids and liquid produces a solid phase that is high in phosphorous and fiber and is easily transported. This material is commonly used as animal bedding or as an ideal soil amendment. More novel and experimental uses include the production of construction products such as wall board, decking, and greenhouse pots. The liquid fraction is high in nitrogen and low in phosphorus thus enabling irrigation of fields that may be phosphorus limiting.

To maximize biogas production and the potential return on renewable energy sales, conditions within the digester must be optimized. One important factor is the pH, the amount of acidity in the manure. This is often not an issue with animal manure as it is most often neutral (which also makes it an excellent source for blending with other wastes). Temperature is another important factor. Digesters must be heated in colder climates as the microbial community requires a temperature of about 100oF. The amount of water used in a farm’s manure management is also influential. Excess water results in higher costs to heat digesters and reduces the concentration of organic material. Only enough water should be used to clean barns and make manure transferable.

The amount of organic material is often measured as the biochemical oxygen demand (BOD) or chemical oxygen demand (COD). To increase the BOD and COD (and increase gas production), manure can be blended with more concentrated wastes such as those from food processing, ethanol (stillage), and biodiesel (glycerin) plants.

General Estimates of Cost

A 1400-pound lactating cow produces about 24 gallons of manure/day requiring at least 266 gallons of digester space (1). The net energy produced is about 18,000 BTU/day per dairy cow or 385 Net kW-hour/day per dairy cow, when 35% of this energy is used to heat the digester (2). The U.S. Environmental Protection Agency estimates a digester costs between $200 and $700 per 1000 lbs. live weight (3) while another estimate says $150 to $500 per cow with maintenance between $11,000 and $51,000 annually (4).

Efforts are being made to reduce these costs to the individual farmer. The idea of a centralized anaerobic digester is one that is currently being investigated. A centralized digester shared by farmers reduces upfront costs. However, issues with transfer of the manure, as well as maintenance and ownership, must be overcome. Also, before implementing a large-scale system, it is recommended that farmers have their manure tested for biogas potential.

Biogas-Potential Assay

Michigan State University’s Department of Biosystems and Agricultural Engineering developed an anaerobic laboratory assay designed to determine if further development studies are warranted. This test, conducted in a laboratory, examines the physical characteristics of the manure (COD, % solids, pH, and other important characteristics) both before and after digestion, and biogas production in a simulated digester. An anaerobic respirometer consisting of closed vessels connected to an automated gas measuring system is used for the assay. Above is a photograph of the laboratory anaerobic respirometers used for biogas determination. These tests are advantageous because they require very little manure (less than 5 gallons), take relatively little time (about a month), are inexpensive, and are easily customized to different mixtures of manure and other agricultural residuals. However, it should be noted that this is only a preliminary test and that the decision to implement an anaerobic digester should not be made based on this test alone. In fact, if this initial test is favorable towards anaerobic digestion, larger pilot-scale studies should be considered. The detailed protocol for this assay can be found at the “Alternatives for Food Processor’s Wastewater” website.

Conclusions

Anaerobic digestion is a natural biological process that offers many environmental protection advantages. Costs can often be offset by the recovery of byproducts when a digestion system is properly designed and managed. To safely and efficiency maximize gas production and maximize environmental benefits training is needed and dedication required. Running a digester is as much an art as a science.